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Functional Plant Science and Biotechnology ©2007 Global Science Books
False Satiation:
The Probable Antiherbivory Strategy of Hoodia gordonii
Simcha Lev-Yadun* • Nitsa Mirsky
Department of Biology, Faculty of Science and Science Education, University of Haifa - Oranim, Tivon 36006, Israel
Corresponding author: * [email protected]
ABSTRACT
The succulent, thorny, South African desert plant Hoodia gordonii and several other species of the genus, as well as Pectinaria maughamii were found to be appetite suppressants and thirst quenchers. The appetite-suppressant drug from Hoodia species increases the content
of ATP by 50-150% in hypothalamic neurons, thus activating an anorectic mechanism. We propose that these plants have evolved such
compounds as a defense against herbivory, causing false satiation. We hypothesize that the same principle operates in other plants, such as
Thaumatococcus daniellii that produces nature's sweetest known molecule (the protein thaumatin). Hoodia and similar plants thus express
a previously unknown mechanism of anti-herbivore defense – false satiation.
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Keywords: defense, herbivory, Hoodia, satiation
Because plants are sessile and they are the main producers
of terrestrial biomass, they suffer from the constant
pressure of various types of herbivory by animals. In response, plants have evolved a rich arsenal of anti-herbivore
defense mechanisms, e.g., chemicals (Biere et al. 2004),
spines (Grubb 1992), aposematism and defensive coloration
(Lev-Yadun 2001; Lev-Yadun et al. 2004; Lev-Yadun
2006), mutualism with ants (Huxley and Cutler 1991) and
attracting predators of herbivorous insects (Kessler and
Baldwin 2001).
A succulent, thorny, cactus-like plant from the arid
parts of South Africa, Hoodia gordonii, a member of the
Asclepiadaceae, and several other species of the genus, as
well as Pectinaria maughamii (Archer and Victor 2003),
were found to be appetite suppressants and thirst quenchers.
These plants have been used by the indigenous San people
during their field trips for generations to overcome hunger
and thirst. The Council for Scientific and Industrial
Research in South Africa patented the appetite-suppressant
drug P57 from Hoodia species, and Hoodia extracts are
orally prescribed to treat human obesity. The putative
active component is a trirhabinoside, 14-OH, 12-tigloyl
pregnane steroidal glycoside (Mw = 1008). The compound
increases the content of ATP by 50-150% in hypothalamic
neurons, thus activating an anorectic mechanism (MacLean
and Luo 2004). Feeding lean and obese rats with the
Hoodia extracts resulted in weight loss in both groups
(Tulp et al. 2001, 2002).
The broad public interest in these plants due to their
commercial potential in the diet industry is obvious, but
there seems to be a special scientific issue involved, i.e., the
reason for the evolution of this trait. We propose that these
plants have evolved such compounds as a defense against
herbivory. Several types of plant chemical defense are
already known: Poisoning of herbivores by toxins is a very
common defense (Janzen 1971; Bryant et al. 1991; Kursar
and Coley 2003), inhibition of digestion by various molecules such as tannins (Green and Ryan 1972; Bryant et al.
1991), olfactory aposematism (Eisner and Grant 1981) and
attracting predators and parasitoids of herbivores by various
volatiles (Kessler and Baldwin 2001). Real herbivore
satiation by production of exceptionally large numbers of
Received: 30 November, 2006. Accepted: 10 December, 2006.
seeds in certain mast years is a well-known plant defense
from seed predation (Janzen 1976; Kelly and Sork 2002).
Satiation may influence the diet of animals considerably
(Pyke 1984). Causing false satiation, as happens with
Hoodia and certain other plants, might be no less rewarding
than other types of defense. The benefits for the plant are
clear: in the short run, an herbivore that grazes on the plant
will eat less and will damage the plant less. Herbivores that
graze such plants may become weaker in time because they
will repeatedly eat less and thus likely be prone to disease
and predation or suffer from lower reproduction. We
hypothesize that the same principle operates in other plants,
such as Thaumatococcus daniellii that produces nature's
sweetest known molecule (the protein thaumatin) (Cagan
1973; Waliszewski et al. 2005), and probably via other molecules in other plants that are frequently identified as producing intensive sweeteners (Bassoli 2004). Although plants
regenerate better than animals after being partly eaten (Ohgushi 2005) and may benefit more at the individual level
from such a trait, theoretically, animals (especially clonal or
social) might also employ the same principle of inducing
false satiation in their predators to reduce predation. We
thus propose that Hoodia and similar plants express a previously unknown mechanism of anti-herbivore defense – false
satiation.
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